Abstract
For high strength steels, a characteristic fatigue behavior, which S-N curve comes down again in the long life region of N > 107 cycles, was reported by many researchers. Specifically, for high strength steel, the fatigue limit is temporal value because of the fish-eye fracture in the long life region over about N = 107 cycles. However, fish eye fracture does not appear for medium strength steel in super long life range. In this paper, cantilever-type rotational bending fatigue tests were carried out for quenched and tempered Cr-Mo steel (JIS: SCM435) up to 109 cycles at room temperature in air. The S-N curve showed a horizontal part in long life region over 107 cycles. In addition, fish- eye fracture was not observed and for all specimens, the crack initiation occurred from the specimen surface. The fatigue fracture behavior of SCM435 in super long life was discussed based on optical microscopy observation and scanning electron microscopy observation.
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